Most commercial adhesives contain chemicals that are harmful to the environment. The development of safe biobased adhesives holds the promise to attenuate these harmful effects. Strong protein (polypeptides) adhesives have been isolated from the phenol gland of mussels of the genus Mytilus (Papov et al., 1995; Waite, 1983; Yamamoto et al., 1983; 1990; 2000; Yu and Deming, 1998). Burzio and Waite (2000) showed that the adhesive strength of the principal mussel protein, mefp1, relies on DOPA-DOPA cross-links. The adhesive protein consists of tandem repeats of the consensus sequence AKPSYPPTYK (SEQ ID NO: 1) that have undergone posttranslational hydroxylation of the two tyrosines to DOPA.
These materials continue being studied since they can be prepared from a renewable resource, are biocompatible and biodegradable, and possess unique self-assembling properties.
Such protein adhesives have many potential applications, including tissue adhesives and bonding agents for implants and mucoadhesive drug delivery. On the other hand, abundant adhesive proteins are available from renewable resources and agricultural by-products such as soybean proteins. Developed in the 1920s, plant (soy) protein-based adhesives were used mainly in wood based products (e.g. particle board and plywood). Recently, modified soy protein-based adhesives have improved adhesive strength and water resistance (Hettiarachchy et al., 1995; Huang and Sun, 2000a,b; Sun and Bian, 1999; Kalapathy et al., 1995, 1996; Zhong et al., 2002). While the amino acid sequences of numerous adhesive proteins are known (Adachi et al., 2001; Nielsen, 1985; Staswick et al., 1981; 1984), the molecular interactions of protein binding on wood surfaces are poorly understood.
One molecular interaction is based upon the characteristic of the soy protein-based adhesives is the presence of long hydrophobic stretches of amino acids. Electrostatics do not appears to be important. The adhesive strength of wood composites bonded with isolated proteinaceous bioadhesives appears to be directly correlated to the number of the amine functional groups in the adhesive (Yamamoto et al., 1995). Also, peptides and protein-based polymers can be designed for a specific function using the biologically available native amino acids where the side chains are chemically modified with functional groups (Lee et al., 2001; van Hest and Tirrell, 2001; Yamamoto et al., 1995; Yamamoto et al., 1990).
What is needed is a rational design approach for making protein adhesives where the contributions of the different amino acid side chains, either singly or in groups, may be assayed for adhesive properties, secondary structure, and modes of action at the molecular level.